1. Field of the invention
The present invention relates generally to light emitting devices, and in one of its aspects, to a light emitting device that has a substantially constant output power.
2. Description of the Prior Art
Normally a light emitter such as a light emitting diode is placed in a circuit that supplies it with a relatively constant current. Most such devices, such as gallium arsenide light emitting diodes, have continually decreasing output power for a given current over the life of the device.
In the past, both a light emitting chip and an integrated circuit including a light sensor have been mounted on a single header in such a way that the light sensor senses the light emitted by the light emitter. Such combinations have occurred in optical isolators, and were used only for electrical isolation.
Light emitting devices such as light emitting diodes have a variety of uses, some of which would be better served if the output of the light emitting devices could be made fairly constant during periods of use over a long period of time. One such use is for determining the reflectivity at a given moment of a surface of variable reflectivity. The light emitting device is positioned near the variably reflecting surface and turned on. A light sensor is placed in proximity to the light emitter in such a way that the sensor senses light reflected from the variably reflecting surface. In order to know what the reflectivity of the surface is based upon the amount of light sensed by the sensor, the amount of light emitted by the light emitter must be known. It is difficult to predict the amount of light emitted from a conventional light emitting device, making it difficult to determine the reflectivity of the surface.